The human foot. A companion to clinical studies

The Human Foot

Leslie Klenerman and Bernard Wood

The Human Foot A Companion to Clinical Studies With 85 Figures

With contribution by: Nicole L. Griffin, MS Hominid Paleobiology Doctoral Program George Washington University Washington DC, USA

Leslie Klenerman, MBBCh, ChM, FRCS Emeritus Professor of Orthopaedic and Accident Surgery, The University of Liverpool, Liverpool, UK

Bernard Wood, MBBS, PhD, DSc Henry R. Luce Professor of Human Origins, Center for the Advanced Study of Hominid Paleobiology, Department of Anthropology, George Washington University, Washington DC, USA

A catalogue record for this book is available from the British Library Library of Congress Control Number: 2005925191 ISBN-10: 1-85233-925-X ISBN-13: 978-1-85233-925-8

Printed on acid-free paper

© Springer-Verlag London Limited 2006 Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency. Enquiries concerning reproduction outside those terms should be sent to the publishers. The use of registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant laws and regulations and therefore free for general use. Product liability: The publisher can give no guarantee for information about drug dosage and application thereof contained in this book. In every individual case the respective user must check its accuracy by consulting other pharmaceutical literature. Printed in the United States of America. 987654321 Springer Science+Business Media springeronline.com

(SPI/EB)

Preface The appendages at the end of our forelimbs tend to attract the evolutionary and clinical limelight, but our feet are as important as our hands for our survival and success as a species. We tend to take them for granted, yet the many millions of modern humans who run either competitively or for recreation, or who play sports such as soccer, tennis, and badminton, or who ride, or dance, or swim, or climb, or who stand and walk as part of their work, all depend on their feet. We submit them to unreasonable loads, and expect them to survive our pounding them on hard pavements. We also add insult to injury by squeezing them into fashionable but uncomfortable footwear which does not conform to the shape of the foot. All this means that many professionals make their living caring for our feet. Worldwide many hundreds of thousands of professional people spend most of their working life looking after the foot. They include orthopaedic surgeons, rheumatologists, diabetologists, orthotists and prosthetists, physical therapists, and podiatrists of whom there are at least 15,000 in the United States of America alone. In the English language there are two classic books about the foot, both by anatomists. In 1935 the American anatomist Dudley Morton wrote the first edition of The Human Foot, and in Great Britain Frederick Wood Jones’ seminal book, Structure and Function as Seen in the Foot, was published in 1944. But since these pioneering efforts great strides have been made in our understanding of the evolution and function of the foot. This book is not intended as a replacement for the Morton and Wood Jones monographs, but instead it is designed to provide contemporary users and healers of the foot with some context about feet. Neither is it intended to be a clinical textbook. Instead, we hope it will appeal to a wide constituency, including the professionals who care for feet, and to the many categories of `users’, such as long distance runners and soccer and tennis players who depend on their feet to take them where they want to be, whether it is the finishing line of a marathon, or a place on a field or a court from where they can kick the winning goal, or play the decisive shot. This book, the combined effort of an orthopaedic surgeon and an anatomist/ palaeoanthropologist, is not intended to be comprehensive but to stimulate readers to go off on their own voyages of discovery. We have subtitled it A Companion to Clinical Studies because we hope that clinicians will find within its covers information that will deepen their understanding of the function and evolutionary history of this intriguing structure. Writing any book always requires help from others. LK thanks the many friends and former colleagues who provided assistance. These include David Bowsher, Director of Research at the Pain Relief Foundation, Liverpool; Professor Robin Crompton of The University of Liverpool; Professor Adrian Lees of Liverpool John

vi

Preface

Moore’s University; Professor Phillip Tobias at his alma mater, The University of the Witwatersrand, Johannesburg; Susan Barnett, senior research fellow of The University of the West of England and member of the Foot Pressure Interest Group; Peter Seitz of Novel Gmbh, Munich; and Roger Mann, Oakland, California. All were invaluable sources of information and advice. In addition, John Kirkup, a retired orthopaedic surgeon in Bath, was LK’s advisor on history, and Drs. Harish Nirula and Harry Brown of the Artificial Limb Fitting Centre at the Wrexham Maelor Hospital gave generously of their time and expertise to provide information about amputations and prostheses. Alun Jones and Andrew Biggs of the Photographic Department at the Robert Jones and Agnes Hunt Orthopaedic Hospital, Oswestry were a bastion of support and dealt efficiently with all the illustrations. My secretary Anne Leatham cheerfully coped with the long hours involved in typing draft chapters and references. Last, but not least, I am grateful for the support and constructive criticism which were always available from my wife, Naomi, and from my younger son, Paul. BW is particularly grateful to four of his teachers. Eldred Walls taught him the anatomy of the foot, Michael Day introduced him to palaeoanthropology, Owen Lewis emphasised the importance of rigorous comparative anatomy, and Leslie Klenerman taught him orthopaedics. BW and NG are also especially grateful to Brian Richmond and Elizabeth Strasser who reviewed drafts of Chapter 1; any errors that remain are due to our intransigence. We appreciate help from Phillip Williams and Matt Skinner for generating figures and tables for Chapters 1 and 2. We thank Pilou Bazin for providing translations of articles in French. We are also grateful to the many experts, especially Osbjorn Pearson, Jennifer Clack, Will Harcourt-Smith, and Susan C. Antón, who patiently answered our questions and enquiries, as well as to the authors and publishers who allowed us to include their illustrations in this volume. BW thanks the Henry Luce Foundation for support and NG wishes to acknowledge the support of an NSF IGERT Graduate Studentship Award. Leslie Klenerman, Bernard Wood

Contents

1. Early Evolution of the Foot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . From Fins to Feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Foot Diversity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Primate Feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 4 10

Pads, Claws and Nails . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ancient Primates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Miocene Primates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

15 16 18 22

2. Recent Evolution of the Human Foot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hominin Evolution: An Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

27 27

Evolution of Bipedalism within the Hominin Clade . . . . . . . . . . . . . . . . . . . . . .

34

Review of Individual Hominin Fossil Taxa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

37

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

70 71

3. How the Foot Works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Subtalar Joint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Midtarsal or Transverse Tarsal Joint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . How the Longitudinal Arch Works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flat Feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vibrations in the Human Skeleton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

81 81 83 87 88 89 92 92

The Importance of the Toes in Walking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Sole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

95 96

Vertical Clinging and Leaping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Suspension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quadrupedalism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bipedalism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Defining Hominins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Organising the Hominin Fossil Record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Scenarios Favouring Selection for Bipedalism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Primitive Hominins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Archaic Hominins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Megadont Archaic Hominins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Archaic Homo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anatomically Modern Homo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Shock Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11 12 13 14

27 28 30

36

38 40 52 56 69

94

viii

Contents

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

99 99

4. The Development of Gait . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Prenatal Movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 The Infant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 Newborn Stepping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 Gait . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Running . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Centre of Gravity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Changes in the Limbs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Foot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Common Variations Which Disappear with Growth . . . . . . . . . . . . . . . . . . . . . Mechanical Work in Walking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bound Feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

109

109 110 111 113 113 114 115 116

5. The Measurement of Footprints (Pedobarography) . . . . . . . . . . . . . . . . . 117 Force Plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 Pressure Transducers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 Modified Force Plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 Laboratory Practice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 Loading of the Normal Foot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 Assessment of Foot-pressure Patterns from a Pedobarograph Using Techniques of Imaging Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 The Effect of High and Low Heels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 The Effect of Obesity on Patterns of Foot Pressure . . . . . . . . . . . . . . . . . . . . . . 130 Shear Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 Plantar Pressures in the Great Apes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 6. The Foot in Action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 The Armless or the Foot as a Hand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 Footedness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 Running . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 Ballet Dancing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 The Foot in Microgravity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 Tightrope Walking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 Firewalking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 Kicking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 Swimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

Contents

ix

7. Amputations and Prostheses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 Degree of Loss: Simple to Radical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 Amputation of the Great Toe (Hallux) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Amputation of All Toes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Partial Amputations of the Foot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Disarticulation at the Ankle (Syme’s Amputation) . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biomechanics of Midfoot and (Syme’s Amputation) . . . . . . . . . . . . . . . . . . . . . . . . . . Calcanectomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

157 158 158 159 160 161

Prosthetic Feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

161

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

171

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

175

Energy Return Feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phantom Limb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

167 170

1 Early Evolution of the Foot The history of life can be best understood using the analogy of a tree. All living things, be they animals, plants, fungi, bacteria, or viruses are on the outside of the tree, but they are all descended from a common ancestor at its base. The evolutionary history of all these living forms is represented by the branches within the tree. Modern humans are at the end of a relatively short twig. There is reliable genetic evidence to suggest that our nearest neighbor on the Tree of Life is the chimpanzee, with another African ape, the gorilla, being the next closest neighbour. The combined chimp/human twig is part of a small higher primate branch, which is part of a larger primate branch, which is just a small component of the bough of the Tree of Life that includes all animals (Figure 1.1). This chapter looks into the branches of the Tree of Life to reconstruct the ‘deep’ evolutionary history of modern human feet. Our feet are unique. No other living animal has feet like ours, but as we show in the next chapter some of our extinct ancestors and cousins had feet that were like those of modern humans. What type of animals showed the first signs of appendages that eventually gave rise to primate feet? What can we tell about these distant ancestors and cousins that will help us make sense of the more recent evolutionary history of modern human feet? We trace the emergence of simple, primitive, paired limbs and then examine how selective forces have resulted in the various types of foot structures we see in some of the major groups of living mammals. Our focus then narrows to the early evolutionary history of the Primate order so that in the next chapter we can concentrate on the more recent evolutionary history of the human foot.

From Fins to Feet The first animals to develop appendages, the precursors of primitive limbs of land vertebrates, lived in seas, lakes, and rivers. The pioneers of land were called tetrapods and they may have ventured out of water as early as the Upper Devonian period, approximately 370 million years ago (Mya). However, due to the paucity of fossils, palaeontologists have difficulty piecing together why tetrapods developed limbs in the first place and what caused this terrestrial radiation. There is evidence that seeking a home on land did not initially drive the growth of limbs and feet. Acanthostega, an early member of the lineage that includes terrestrial vertebrates, possessed fore- and hindlimbs effective for propulsion in water, but

Chapter co-written by Nicole L. Griffin.

2

The Human Foot

Modern humans

Chimpanzees

5-7 Chimpanzees and modern humans

Gorillas

7-9 African great apes

Asian great apes

14-15 Great apes

Lesser apes

16-19 Apes

Monkeys

25-30 Monkeys and Apes

Other Primates

35-40 Primates

Other Mammals

c. 55 Mammals

Other Tetrapods

c. 250 Tetrapods

Other Vertebrates

c. 400 Million of years

Vertebrates

Figure 1.1. The branches of the vertebrate part of the Tree of Life leading to modern humans, including the approximate ages of the major branching points.

its wrists and ankles were not strong enough to provide support for locomotion on land [1]. Some vertebrate palaeontologists argue that the earliest tetrapods emerged in shallow swamps. This theory is supported by 365-million-year-old tetrapod trackways in Ireland [2]. Scientists believe the tetrapod who made these tracks was not walking on dry land, but was using its limbs partly as supports and partly as paddles to make its way through shallow water. They argue that if it had been walking on dry land, its tail would have left an imprint. However, it is possible Acanthostega is a representative of a tetrapod line that had adapted to living on the land and returned to the water. Some researchers think that a creature contemporaneous with Acanthostega named Hynerpeton, which has a forelimb strong enough to support it on dry land, may have been the one of the first tetrapods [2].

Early Evolution of the Foot

3

One reason put forward to explain the adaptive radiation of tetrapods is that they moved onto the land during unusually dry spells. If a water hole was drying up it made sense to venture onto the land to find a different, more reliable source of water [3,4]. However, the behaviour of the modern lungfish argues against this explanation, for when there is a drought it simply buries itself in the mud. This prevents desiccation, and allows it to survive until wetter conditions prevail. The ‘water seeking’ hypothesis is also weakened by the existence of aquatic creatures that can move across the land without possessing limbs. Eels, for example, which have only very abbreviated fins, make substantial excursions onto the land during the spawning season. Indeed, digitlike appendages are not exclusive to terrestrial vertebrates, for they are found in the Sargassum frogfish (Figure 1.2), an obligate aquatic dweller [1]. Currently, palaeontologists are more inclined to favour a scenario where either, or both, resource availability and predation were the driving forces behind the evolution of limbed tetrapods. It is possible that during the Upper Devonian the combination of an increase in aquatic predators and resource competition forced tetrapods to venture onto the land where their vulnerable larvae or juvenile offspring would be safer, and where food, in the form of arthropods, was more plentiful [1,5]. For the tetrapod, life on land required a change in both the respiratory system and the appendages. The replacement of gills with lungs allowed land-dwellers to inhale air and exchange oxygen for carbon dioxide. A nonaquatic environment exposed tetrapods to gravitational forces, and the development of fore- and hindlimbs lifted the thorax off the ground [6]. Not only did paired limbs replace paired fins, but also the relationship between the pectoral and pelvic girdles changed dramatically. Like their osteichthyan (bony vertebrate) ancestors, most liv-

Figure 1.2. The Sargassum frogfish remains still in the presence of its prey by grasping vegetation with its digit-like appendages. (Reprinted with permission from Clack JE. Gaining Ground: he Origin and Early Evolution of Tetrapods, page 103, Indiana University Press 2002.)

4

The Human Foot

ing bony fish have pectoral fins that are larger than their pelvic fins [1]. However, when the tetrapods shifted to the land in most cases their pectoral limbs, or forelimbs, became smaller than their pelvic, hindlimb, counterparts [1,5]. In addition, with the establishment of a direct connection between the hindlimb and the vertebral column [5–7] and the fusion of the os coxa [5], the tetrapod pelvic girdle became a stable and powerful lever for transferring force from the hindlimb to the axial skeleton. Also, unlike in living fish in which each half of the pelvic girdle is anchored independently to the body wall, both halves of the tetrapod pelvic girdle meet in the midline to form a pubic symphysis, therefore providing more structural support. Other characteristics of the hindlimb unique to tetrapods include a deeper socket where the femoral head articulates with the pelvic girdle to create a more stable joint [1,8]. In addition, the lowermost vertebrae unite with the equivalent ribs to form a wide sacrum onto which leg and tail muscles insert [1,8]. The proximal regions of both the forelimbs and hindlimbs of tetrapods retain the skeletal formulae seen in the pectoral and pelvic fins, respectively, of bony fish [1]. In the tetrapod, the proximal segment (stylopodium) of the forelimb and hindlimb consists of the humerus and femur, respectively. The distal segment (zeugopodium) consists of the radius and ulna in the forelimb, and the tibia and fibula in the hindlimb. However, it is at the distal ends of the limbs where the distinctiveness of tetrapods is evident. In humans and most other tetrapods, the autopodium consists of carpals, metatcarpals, and phalanges in the forelimb and tarsals, metatarsals, and phalanges in the hindlimb. Very few lobe-finned fish (the closest fish relatives to tetrapods) possess homologues to tetrapod metacarpals and metatarsals and respective phalanges [1,9–11]. Although the standard number of digits in contemporary tetrapods is five, with exceptions being birds who commonly have four toes and horses who have one toe, investigators are unsure why pentadactyly is the most widespread digital formula in living tetrapods and how many times it evolved (Figure 1.3). Pentadactly may have not been the primitive condition because the early tetrapod, Ichthyostega, had seven digits [1]. The standard early tetrapod pentadactyl hindlimb was arranged in three rows (Figure 1.4). The proximal row consisted of the tibiale, the intermedium, and the fibulare. The middle row consisted of four centrales [12]. The distal row comprised five tarsals and a prehallux along with their respective metatarsals and phalanges [12,13].

Foot Diversity About 90 million years after the appearance of the first primitive tetrapods, around 310 Mya, the earliest members of the reptilian lineage were the first animals to lay eggs on land. The morphology of these early reptiles suggests they were more committed to a terrestrial life style than either their amphibian ancestors or modern-day amphibians. The propulsive thrust of the hindlimb

Early Evolution of the Foot

5

Figure 1.3. Pedal digit number varies among extinct and living tetrapods. lchthyostega possessed seven digits (A). (Coates, M.I. and Clack, J.A. 1995. Romer’s Gap - tetrapod origins and terrestriality. In Arsenault, M., Lelivre, H. & Janvier, P., (Eds) Proceedings of the 7th International Symposium on Early Vertebrates. Bulletin du Muséum national d’Histoire naturelle, Paris, pp. 373-388. © Publications Scientifiques du Muséum national d’Histoire naturelle, Paris.) While contemporary horses have one digit (B), chickens have four digits (C), and modern humans have five digits (D). (Reprinted with permission from Adams and Eddy [102, pp. 235–238].)

fibula

tibia

int fib

tib c

c

P

Figure 1.4. The lower limb of an early tetrapod, Trematops milleri. tib, tibiale; int, intermedium; fib, fibulare; c, centrale; p, prehallux; 1-5, tarsals; l-V, metatarsals. (Reproduced with permission from Lewis OJ. The Homologies of the Mammalian Tarsal Bones. J Anat. 1964;98:195-208. Blackwell Publishing Ltd.)

1 I

c

2 II

c

4

3 IV III

5 V

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The Human Foot

was improved by increasing the scope for muscle attachment on the sacrum. In addition, modifications to the foot allowed a more rapid locomotion in reptiles compared to their predecessors [6]. During the early evolution of the reptiles, the number of centralia became reduced [12]. Having already appeared in some of their primitive tetrapod ancestors, the reptilian calcaneum and talus (also called the astragalus) were recognizable entities. It is accepted that the calcaneum developed from an elongated fibulare [12,14], but the origin(s) of the talus are still debated. Gegenbaur [15] suggested the talus was formed from the union of the tibiale and intermedium, and others argued that the intermedium is the sole precursor to the talus [12,16–19]. Steiner [20] made a third suggestion, that the two centrales coalesced with both the tibiale and intermedium to form the talus. On the other hand, Berman and Henrici [21] have identified a fossil talus showing sutural lines demarcating where the intermedium, tibiale, and proximal centrale have fused. This supports Peabody’s [22] hypothesis that the talus was formed by the union of these bones. It was not until the Triassic Period (~230 Mya) that limbs began to be situated beneath the body. This change, which improved bodily support and made locomotion more efficient, is especially evident in the postcranial morphology of bipedal archosaurs, one group of the Triassic reptiles (Figure 1.5). Because their forelimbs were no longer involved in supporting the body, their forelimbs and Reduced pectoral girdle Expanded ilium; rod like pubis and ischium

Grasping hands

Strengthened ankle joint

Elongate hind limbs; joints remodeled for erect, bipedal posture

Figure 1.5. The body plan of a bipedal archosaur. (Reprinted with permission from Radinsky L, The Evolution of the Vertebrate Design, page 109, Chicago: University of Chicago Press 1987.)

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7

forelimb girdle became more gracile, and the pelvic girdle became more robust and expanded its contact with the vertebral column. This helped to transmit body weight and increased the area available for the attachment of the muscles involved in locomotion. The hindlimbs were also lengthened, and were brought closer to the midline. A hinge joint was established where the talus and calcaneum meet with the fibula and tibia. This would have improved the transmission of body weight and increased the potential for fore–aft motion of the hindlimb, thus saving energy and increasing speed of locomotion. Bipedal archosaurs probably walked on their digits; their tarsals did not come in contact with the ground [6]. Pterosaurs evolved from archosaurs and, as their name suggests, they were specialised for flight. Their forelimbs were transformed into wings, and their hindlimbs retained the form of their archosaur relatives. The ancestry of modern birds can also be traced back to the archosaurs. Living birds stand upright, with most of their weight in front of the attachment of the hindlimbs. The femur and tibia are oriented so that the joint between them more or less forms a right angle. This prevents birds from falling forward [23,24]. Birds stand on their toes, with three digits pointing forwards and one back. Their metatarsals are elongated and are fused with the tarsal bones. This combined elongated tarsometatarsus forming a simple hinge joint enabled birds to take longer strides [14,23]. Whereas some archosaurs became bipedal and/or were specialised for flight, others turned to an aquatic life (Figure 1.6). The appendages of these marine reptiles were customised into paddles. One group, the plesiosaurs, maintained a

Femur T

Figure 1.6. The paddlelike hindlimb of Plesiosaurus increases the number of phalanges in each ray. T, tibia; Fi, fibula; c, centrale; a, astragalus; cal, calcaneus; 1-V, tarsals. (From Carroll RL. Patterns and Processes of Vertebrate Evolution, page 252, 1997. Reprinted with the permission of Cambridge University Press.)

Fi

c

a

1

2-3

cal 4

v

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The Human Foot

five-digit autopodium and added several more tarsals and phalanges [24]. Marine reptiles, such as icthyosaurs, also increased the number of their foot phalanges, but reduced their autopodium to three digits. These creatures became so specialised for life in water that it is probable they were incapable of venturing beyond the shoreline [6]. Mammals evolved about 225 Mya from a group called the therapsids, or mammallike reptiles. In mammals, the navicular is recognizable, but the fifth primitive tarsal bone is lost. The origins of the navicular and the fate of the fifth tarsal are contested. Schaeffer [25] and Romer [26] argued the navicular is a homologue of one of the centrales, but Steiner [20] suggested the navicular was formed from the amalgamation of two centrales. The majority of studies suggest that the therapsid navicular was formed from the union of the tibiale and centrale [12,16,17,27–30]. In primitive mammals, only four tarsals, the three cuneiforms and the cuboid, articulate with the metatarsals. Steiner [20] acknowledged the fifth tarsal as lost and identified the fourth as the cuboid. In contrast, Lewis [12] suggested that the cuboid is formed from the fusion of the fourth and fifth tarsals. With the emergence of mammals, changes to limb morphology led to even more types of locomotor repertoires. The most revolutionary of these morphological changes were the superimposition of the talus over the calcaneum and the elimination of the articulation between the fibula and calcaneum. Thus, the weight-bearing function of the fibula was reduced [31,32]. Other changes in the hindlimb morphology included moving the feet even closer to the body and knees that point forward. At the hip and ankle joints the elongation of two bony processes, the greater trochanter of the femur at the hip, and the posteriorly-projecting calcaneum at the ankle (the Achilles heel), increased the mechanical efficiency of muscles. The greater trochanter of the femur improved the moment arm of the gluteal muscles, and at the ankle the elongated calcaneum improved the moment arm of the muscles attached to the Achilles tendon [6]. The main bony elements of the hindlimb, the stylopodium and zeugopodium, are relatively conservative and have retained their primitive morphology throughout most of the evolutionary history of the tetrapods. In contrast, the autopodium at the distal extremity of the hindlimb has become highly specialised, especially within mammals. As outlined by Hildebrand [33], we recognize and introduce five locomotor groups. These are not grouped according to phylogenetic criteria, but according to shared mechanical demands. First are runners and jumpers; second, diggers and crawlers; third, climbers; fourth, swimmers and divers; and fifth, fliers and gliders. Members of the fast-running mammalian group include ungulates, and hopping rodents and marsupials. Among the ungulates, the even-toed artiodactyls, such as deer, sheep, and elk, have evolved a foot skeleton that comprises only the third and fourth metatarsals, also called metapodials, that are fused together to form the cannon bone. The phalanges associated with those metatarsals are each encased in a hoof. The other main ungulate group, the odd-toed perissodactyls,

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9

which includes horses, have just a single metapodial, and their phalanges are encased in one hoof. The cannon bone and the single metapodial provide sufficient strength, and their light weight allows their owners to run quickly [6]. The distal segments of the hindlimb in hopping mammals are differently adapted for speed. The length of the hindlimb is proportional to the distance of muscular force used during jumping. In turn, this distance directly correlates to the height and distance of the jump. To lengthen the hindlimb, some mammals, such as rodents, kangaroos, and primates have elongated their ankle bones [6]. Members of the digging and crawling group have reduced their forelimbs and hindlimbs so that they do not obstruct movement through narrow burrows. Some burrowers such as the tuco-tuco rodent use their forefeet and hindfeet to push dirt out of the way. The foot pads of these creatures have been widened for more effective use [33]. The third group consists of climbers. For most, the feet have been modified to grip the substrate. For example, opposums and most primates have divergent first toes. As a secondarily-adapted climber, the Central American porcupine is able to fold together the medial and lateral sides of its foot to grasp a tree limb. Another approach to grasping is exhibited by the anteater. This creature opposes its heel to make contact with its two toes. Primates, anteaters, and porcupines have pads on their hands and feet for cushioning and to create friction during climbing. Some climbers, such as squirrels, have claws that also aid in adhering to the support [33]. Swimmers and divers, such as whales and dolphins, have evolved streamlined bodies and short limbs. Mammals that lead both a terrestrial and aquatic life, such as sea lions and seals, have converted their fore- and hindfeet into paddles by lengthening their digits and developing webbing between them [6]. The only mammals who became specialised for flight are bats. Their forelimb bones, especially their lateral digits, have become elongated as struts for wings, whereas their hips and ankle joints and their hooklike digits are adapted for hanging upside down [6]. Modern humans are the only living primate, indeed they are the only living mammal, that is an obligatory striding biped. Obligatory bipeds are animals that as adults rely solely on their hindlimbs for support and propulsion when walking on the ground. It is almost certain that this very special type of primate locomotion evolved within the last five to eight million years. The rest of this chapter and the next trace the evolutionary history of modern human bipedal locomotion. Modern humans belong to the order Primates, the suborder Anthropoidea, the superfamily Hominoidea, the family Hominidae, the tribe Hominini, and the genus Homo (Figure 1.7). A more reader-friendly way to express this very technical description is that modern humans are one of the apes, that apes are in turn part of the monkey and ape subgroup of the primates, and that this primate subgroup originated in the distant past, at least 60 million years ago, from the common ancestor of all living and fossil primates. Thus, if we are to understand the evolutionary context

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ORDER

Primates

SUBORDER

INFRAORDER Lemurs SUPERFAMILY

Anthropoids

Prosimians

Tarsiers

Lorises

New world monkeys

Old world monkeys & apes

Squirrel & owl FAMILY Marmosets Howler & spider & tamarins monkeys, capuchins monkeys SUBFAMILY

Baboons, macaques & mangabeys

TRIBE

GENUS

SPECIES

Apes

Old world monkeys

Gibbons

Great apes

Colobines & langurs Pongins

Gorillins

Panins

Hominins

Pongo

Gorilla

Pan

Homo

Orangutans Gorillas

Bonobos Chimpanzees Modern humans

Figure 1.7. Primate taxonomy.

of the modern human foot we need to find out what information comparative anatomy and the fossil record can tell us about a series of common ancestors. These are, in order from oldest to youngest, the common ancestor of all primates, the common ancestor of anthropoid primates (monkeys and apes), the common ancestor of apes, the common ancestor of the African apes including modern humans, and finally the common ancestor of modern humans and chimpanzees. Only then will we be in a position to try to make sense of the human fossil record to see what sort of feet our cousins and immediate ancestors had.

Primate Feet Before discussing the evolutionary history of the primate foot, we first review the feet of living primates so we can better understand how they relate to differences in primate locomotion. This survey of the way living primates have modified the design of the primitive mammalian foot provides a background for our review of the fossil primates ancestral to chimpanzees and modern humans. All primates, except the loris, have five functional digits. Almost all the feet of living primates, with the conspicuous exception of modern humans, have a divergent big toe or hallux [32]. This is an adaptation for grasping. The thumblike

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11

‘saddle’ joint between the hallux and the medial cuneiform combined with the contractions of the toe flexors allow the big toe to meet with the lateral toes in a grasping position. Movements between the talus and the navicular, and the calcaneum and cuboid, allow inversion and eversion movements that help to turn the foot into an efficient grasping organ [34]. The design of the feet of living primates is governed by posture (while resting and eating) as well as by locomotion. Researchers unite locomotion and posture and call the two ‘positional behaviour’ [35]. However, the categories we review below refer in the main to locomotion, although inevitably locomotion and posture are interrelated. It is worth noting that unlike modern humans, all other primates have a locomotor repertoire that includes various kinds of locomotion and their feet reflect this compromise.

Vertical Clinging and Leaping Some primates adopt vertical clinging as their posture at rest and then use leaping in order to move between vertical supports (Figure 1.8). As an adaptation to clinging, indriids have evolved a unique gripping mechanism (Figure 1.9). There

Figure 1.8. Four positional behaviours found in the Primate order (A) vertical clinging and leaping, (B) suspension, (C) arboreal quadrupedalism, (D) terrestrial quadrupedalism, (E) bipedalism. (Reprinted from Primate Adaptation and Evolution, 2nd ed., Fleagle JG, 298-303, Copyright (1999), with permission from Elsevier. Illustrations by Stephen Nash.)

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Figure 1.9. The ‘cleft’ in the indriid foot allows for a wide and secure grip during climbing. (Reprinted from Gebo D. Postcranial Adaptations in Nonhuman primates, pages 175–198, Northern Illinois University Press 1993.)

is a deep cleft between the big toe and the second toe, and the flexor muscles have increased in size [34]. Adaptations for leaping include a long hindlimb built for power at take-off and shock absorption during landing. In tarsiers, superficial flexor muscles are employed for propulsion. Although the tarsal region of the foot varies among leapers, most have an elongated calcaneum and navicular to increase the load arm [35]. Other characteristics common in primates who leap include a large expanded facet on the talus for the fibula in order to increase mobility, and the presence of squatting facets located where the talar body and neck meet. These latter facets suggest that the foot is bent up against the leg (i.e., dorsiflexion) during clinging and prior to leaping [31]. Dorsiflexion and plantarflexion of the foot are promoted by the fusion of the fibula and tibia at the ankle joint in some taxa [35].

Suspension Suspensory primates hang below tree branches using a combination of arm and leg supports (Figure 1.8). The feet of suspensory primates are similar to their hands. The big toe, like the thumb, is well developed and the toe phalanges are long and curved for grasping branches. Sometimes specialised suspensory primates use their feet to grasp a support so that they can use both hands to feed. The long phalanges of the lateral toes hook above the support and the big toe hangs freely or curls around larger branches. Lorises also have a well-developed hallux and strong flexor or toe-curling muscles. Long toes coupled with a wide range of movement at their tarsal joints provide the agility needed to adjust to a variety of supports [34]. The tarsal bones of loris feet have become realigned so that the medially projecting big toe can grasp quite large tree trunks [34,36]. Other modifications of the foot of a suspensory primate include a wide range of movement at the ankle joint, a short calcaneum, and modifications to the short muscles that flex the toes to increase grasping power. Unlike the feet of arboreal clinging and leaping primates, or the feet of terrestrial primates, the feet of suspensory primates are especially mobile [35].

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13

Quadrupedalism The least specialised of the four basic locomotor patterns of primates is quadrupedalism. This form of locomotion uses all four limbs to carry the weight of the body. Primate quadrupedalism can be divided into two categories, arboreal quadrupedalism and terrestrial quadrupedalism. In arboreal quadrupedal primates (Figure 1.8), the foot structure is adapted for grasping, but not to the extent it is in vertical climbers. Asymmetry of the tibio-talar facet allows the foot to be inverted while the toes are used for grasping [35]. Terrestrial quadrupedal primates (Figure 1.8) have robust tarsals and metatarsals. The toes are relatively short and joint mobility is limited. Terrestrial quadrupeds also have a reduced hallux compared to most arboreal quadrupeds because there is less need for grasping [37]. The feet of knucklewalkers (chimpanzees and gorillas) exhibit a mosaic of features characteristic of terrestrial and arboreal quadrupedalism as well as climbing. During knuckle-walking (Figure 1.10), the body weight is transferred through the foot via the long, broad, low transverse arch to the distal metatarsals. The first metatarsal is designed to serve two functions. It acts as a lever during push-off on the ground, and as a vise while moving through the trees [38]. Although both the forelimb and hindlimb are employed in primate quadrupedal locomotion, there tends to be more emphasis on the hindlimb [39]. Contrary to most other quadrupedal placental mammals who locomote using a ‘lateral’ gait sequence, the majority of primate quadrupeds use a ‘diagonal’ sequence [40]. When describing the order in which the four limbs make contact with the substrate, it is conventional to start the gait sequence at the

Figure 1.10. Knuckle-walking gorilla. (Reprinted from Primate Adaptation and Evolution, 2nd ed., Fleagle JG, 247, Copyright (1999), with permission from Elsevier. Illustration by Stephen Nash.)

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point when the right hindlimb is set down on the substrate. In a ‘diagonal’ quadrupedal gait, the next limb to touch the substrate is the left forelimb, then the left hindlimb, and the last limb to contact the substrate is the right forelimb.

Bipedalism The only form of primate locomotion that depends solely on the hindlimbs for locomotion is bipedalism (Figure 1.8). The upper limbs and trunk are involved in bipedalism, but they are used for balance and modulation, not locomotion as such. Modern humans are the only living primates that are habitual bipeds. Most primates use bipedalism infrequently (e.g., gibbons and chimpanzees) and a few specialised leapers hop bipedally when on the ground. In contrast to primate quadrupeds, the centre of gravity at rest in a biped is between the feet. The centre of gravity, however, moves forward when one leg moves forward carrying the body with it [41]. A simple ‘walking cycle’ (i.e., the sequence of events between the times the right heel meets the ground) is divided into the stance phase (i.e., when that foot is on the ground) and the swing phase (i.e., when that foot is off the gound). For 80% of the cycle one or the other hindlimb is in the swing phase and for 20% of the cycle both feet are on the ground. During running the feet do not touch the ground at the same time. With this background, we discuss bipedal striding as it specifically relates to muscle contraction at key joints of the lower limb. At heel strike (the beginning of a walking cycle) flexion has occurred at the hip, the knee is extended, and the leg is laterally rotated. Body weight is moved over the supporting limb by the action of the adductors of the leg in the stance phase. Just before toe-off, body weight is transferred to the hallux. At this point of the stance phase, the hip and the knee are extended. The stance phase is now completed. The swing phase begins with flexion at the hip and knee. The knee then extends when the leg in the swing phase passes the supporting leg. The leg in the swing phase laterally rotates as it prepares for heel strike [42]. To convert the foot from a grasping, handlike appendage to a rigid lever suitable for a bipedal gait, it was necessary to have both a lever arm, the calcaneal tuberosity, and a load arm, in this case a stable tarsus and an adducted robust hallux. Inasmuch as toes were not needed for grasping, they were shortened. The neck of the talus became less obliquely oriented than in quadrupedal primates so that it was aligned with the long axis of the foot. Strong ligaments on the plantar surface bind the tarsals and metatarsals together and function as shock absorbers when the foot takes the weight of the body during the stance phase. A longitudinal arch made of bones and ligaments allows body weight to be transferred from the hindfoot to the forefoot via the lateral side of the foot [42,43].

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15

Pads, Claws and Nails A review of the main types of living foot structures would not be complete without some mention of the superficial soft tissues of the foot. These include the hairless skin pads that protect the underside of the foot and the claws and nails that function as specialised extensions of the foot. In his survey of mammalian feet, Whipple [44] noted there are cutaneous pads on the plantar surface of the foot, one at the tips of each digit and six proximal pads. Four of the six pads are at the junctions of the phalanges and metatarsals. The remaining two, the thenar and hypothenar pads, are located at the medial and lateral aspects of the ankle, respectively. Lorises retain all six of the proximal pads on the foot, and mouse lemurs have done the same, but with the addition of an accessory pad on their first rays (Figure 1.11). Living apes and modern humans have also retained all six proximal pads, but they are only distinguished as clear entities in fetal feet [45]. Later in development they tend to merge and the hairless skin of the pads is covered with fine, whorllike ridges only at the fingertips. These cutaneous pads have two main functional roles. Specialised sensory receptors on the ridges provide increased sensitivity to touch, and the ridges together with the sweat glands help provide friction when the foot is used to grip arboreal supports. In prosimian feet, these specialised epidermal ridges are only found on the apical pads, but in the anthropoids, including humans, the ridges are more widely distributed on the sole of the foot. Cartmill [46] has linked this difference to allometry. In primates, the coalescence of pads scales with body size. The ‘skin’ that surrounds the distal pedal phalanges of mammals is variable in both form and function. Its form includes the massive claws of large-bodied carnivores, the hooves of horses, and the delicate flat nails of primates (Figure 1.12). This relationship between structure and function is very evident when

Apical pad

Accessory pad

Interdigital pad Thenar pad

Hypothenar pad

Figure 1.11. The foot of the mouse lemur (Microcebus murinus) includes an accessory pad on the hallux, a specialization seen in some primates. Note the grooming claw on the second digit. (Martin, Robert D. Primate Origins and Evolution. © R.D. Martin. Reprinted by permission of Princeton University Press.)

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The Human Foot

A

B

C ip

ip

dp

ip

dp Keratinized Nail Plate

dp

Keratinized Hoof Wall

Keratinized Claw

Figure 1.12. The morpology of the mammalian claw (A), nail (B), and hoof (C). ip, intermediate phalanx; dp, distal phalanx. (Reprinted from Hamrick MW. Development and evolution of the mammalian limb: adaptive diversification of nails, hooves, and claws. Evol Dev. 2001;3(5):355–63.)

one considers the role of claws and nails in primates. For example, marmosets use their secondarily-derived claws on their hands and feet to grip a tree trunk when they extract gum [47]. This type of feeding specialisation would be impossible if these primates had flat nails. However, despite specialisations such as those seen in marmosets, all primates have a nail on the big toe. In some prosimians, one or more pedal digits bear clawlike nails specialised for grooming fur. Compared to regular claws, ‘toilet claws’ are longer, more curved, and less sharp (Figure 1.11).

Ancient Primates Functional interpretations of the feet of our distant human ancestors are based on analogies with similar looking feet of living primates and by using morphology to predict the functional demands placed on the feet. This final section traces the evolution of the primate foot during the Paleocene (65–55 Mya), Eocene (55–35 Mya), Oligocene (35–25 Mya), and Miocene (25–5 Mya). Because our focus is on the evolution of the human foot, fossil platyrrhines (New World Monkeys) are not discussed for they diverged from the catarrhines (Old World Monkeys and apes, including humans) approximately 30 to 35 Mya. Little is known about the locomotion of the immediate mammalian ancestors of the primates, but it is most likely that these mouse to rat-sized creatures were at least partly arboreal [48]. Their feet were capable of inversion and eversion and their phalanges look as if they had narrow pointed claws and powerful flexor muscles [31]. The grasping foot is likely to have been the primitive condition for primates, for Steiner [49] noted that during the early stages of mammalian development in utero, the elongation of the medial cuneiform moves the first ray medially so that there is a gap between it and the lateral rays. The first primates appear to have been small nocturnal animals with grasping hands and feet, nails, and a diagonal sequence gait adapted for movement on thin, flexible branches [50,51]. Some suggest that the first primates were Plesiadapiformes (Figure 1.13), which lived during the Paleocene epoch [52]. Although many researchers recognise that Plesiadapiformes share some morpho-

Early Evolution of the Foot

17

Figure 1.13. The plesiadapiform, Carpolestes simpsoni (56 Mya) possessed an abducted nail-bearing big toe capable of opposition, and an ankle designed for leaping. Although Bloch and Boyer [52] suggest these traits as well as others link Carpolestes to Euprimates, Kirk et al. (2003) [104] warn that a clawless opposable hallux is also found in living rodents and marsupials. (Reprinted with permission from Bloch JI, Boyer DM. Grasping primate origins. Science. 2002;298(5598):1606–10. Copyright 2002 AAAS.)

logical traits exclusively with primates [43,53], Plesiadapiformes have unique specialisations that may preclude them from being ancestral to primates [35]. Several key features of the plesiadapiform foot include a talus resting directly on the calcaneum in such a manner as to allow the calcaneum to protract and retract during inversion and eversion of the foot. Researchers also include a welldeveloped calcaneonavicular ligament and flexor digitorum fibularis, a distinguishable talar head and neck, a small or absent talar canal, a subtalar axis occupying an oblique position relative to the long axis of the foot [54], and an opposable hallux bearing a nail [52] as plesiadapiform characters. The Eocene epoch sees the first evidence of Euprimates, a radiation of creatures that are undeniably primates. Eocene Euprimates belong to two families, the Adapidae and the Omomyidae. Fossil adapids and omomyids show several novel features of the foot not found in the feet of the Plesiadapiformes, but they are found in the feet of modern-day prosimian primates that are vertical clingers and leapers [31,55]. These include adaptations of the calcaneum and calcaneal joints to increase the range of rotation of the midtarsal region and the speed of plantarflexion [31]. By the time of the Oligocene epoch (37–25 Mya) researchers are confident they can recognise the postoanial remains early relatives of living anthropoids (monkeys, apes, including humans) in the fossil record [35,53,56]. Two main families of primates are present during this epoch, the Parapithecidae and the Propliopithecidae. The foot of one parapithecid, Apidium phiomense, shares adaptations with the feet of terrestrial anthropoids in that it displays evidence of reduced mobility of the subtalar and transverse tarsal joints and a short calcaneum. However, there are morphological features of Apidium phiomense that are

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also similar to those of living prosimians. The long lower limb and the shape of the upper ankle joint suggest Apidium phiomense was adapted for leaping [57]. Several researchers suggest that a lineage of Propliopithecidae includes the direct ancestors of extant catarrhines (Old World monkeys and apes, including humans) [58–61]. Foot bones, especially the calcaneum, belonging to Aegyptopithecus zeuxis and Propliopithecus chirobates indicate that these two propliopithecids were arborealists. A plantar tubercle on the calcaneum [62] and a calcaneocuboid joint designed for wide rotational capabilities suggest that their feet were designed for suspension and climbing [57]. Curvature of the medial talar trochlear rim provided extensive mobility in the foot of Aegyptopithecus zeuxis. The big toe was abducted, the metatarsals and phalanges were long, and the proximal phalanges curved [57]. Although Aegyptopithecus zeuxis is not directly related to the New World monkeys, it shares some of the same morphology. For example, Aegyptopithecus zeuxis retained a primitive prehallux, a tarsometatarsal sesamoid of the first ray, a feature which is only found in extant primates from the New World [31].

Miocene Primates Approximately 21 genera of ‘hominoids’ have been recovered from layers of strata dating to the Miocene epoch (24–5 Mya) [35]. However, debate centres around which of these fossil primates represent true hominoids and also how each one fits into the family tree of living apes [63]. A probable basal hominoid, Proconsul [60,64] has a foot most comparable to those of living arboreal monkeys [63, and references therein], but also shares some pedal characteristics with great apes [65,66]. Proconsul and living arboreal monkeys have similar talocrural joints and transverse arches [31,65]. For example, the angle formed by the subtalar axis and the long axis of the foot is similar [31]. In addition, Proconsul shares with arboreal monkeys a narrow forefoot and gracile lateral metatarsals and phalanges [31]. Proconsul’s calcaneal facets [67], long hallux [66], and relatively short and robust intermediate phalanges [66] are apelike characteristics. In particular, Proconsul is similar to African apes in its shape and the curvature of the talar trochlea [67] and torsion of the first and second metatarsals [65]. The hallux of Proconsul is strong and robust, capable of powerful grasping and together with a flexible midtarsal joint and long phalanges, the foot of this fossil primate was suited for arboreal quadrupedalism and slow climbing [31,65,66,68]. A later basal hominoid Nacholopithecus has long forelimbs and long pedal digits that indicate it was a forelimb-dominated arborealist [69]. The European hominoid Dryopithecus is a possible last common ancestor of the great apes and humans. This Miocene genus has been divided into several species [70]. Pedal remains are scarce, but a few hand phalanges and a pedal phalanx, as well

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as other postcranial evidence suggest Dryopithecus was a suspensory primate [71]. In particular, the moderately curved intermediate foot phalanx has a mediolaterally broad shaft and well-developed flexor sheath ridges. The distal articular surface indicates that a large range of flexion and extension was possible at this joint. Currently, no fossil has been confirmed as the Miocene ape ancestral to African apes and humans. Therefore, we do not know how this creature moved about. Some researchers suggest that it would have emphasised either climbing and/or suspensory behaviour [72–79], whereas others suggest that the common ancestor of African apes and humans was a terrestrial quadruped [80,81], and several researchers are advocates for the common ancestor being a knuckle-walker and climber [82–88]. Chimpanzees and gorillas are most closely related to modern humans and because they both employ knuckle-walking (chimps more so than gorillas) it is hypothesised that the common ancestor was a knuckle-walker and climber [88]. This hypothesis is supported by Richmond and Strait’s [87] survey of extant hominoids, fossil hominins, and humans. In their report, fossil hominins dating to over 3.5 million years ago show key knuckle-walking wrist traits suggesting that hominins evolved from a knuckle-walking ancestor. Gebo [89] proposed that the most distinguishing feature of the common ancestor of the African great apes and modern humans would be the ability to place the foot in a plantigrade position on the substrate (Figure 1.14). He defines a plantigrade foot as ‘. . . one in which the heel contacts the surface of a terrestrial or arboreal support at the end of swing phase’ [89]. Gebo [89] asserts that all other primates habitually use a semiplantigrade foot posture at the end of the swing phase, although several researchers disagree with him because they have observed New World and other Old World primates using a plantigrade foot position [90,91]. In a semiplantigrade posture, the calcaneum is lifted above the substrate and does not bear any body weight (Figure 1.15). Instead, body weight is borne on the plantar surface of the cuboid, lateral cuneiform, and navicular. To bring the foot into a plantigrade position, the calcaneum occupies an inverted position before contacting the substrate and it maintains this attitude even at midstance when body weight is transferred to the forefoot. The transfer of weight stabilises the forefoot, which is everted [89].

Figure 1.14. The primate foot in a plantigrade position. Only the calcaneum and fifth metatarsal make contact with the ground. (Reprinted from Gebo D. Postcranial Adaptations in Nonhuman primates, pages 175–198, Northern Illinois University Press 1993.)

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Figure 1.15. The primate foot in a semiplantigrade position. The calcaneum is lifted off the ground and the tarsals make contact with the substrate. (Reprinted from Gebo D. Postcranial Adaptations in Nonhuman primates, pages 175–198, Northern Illinois University Press 1993.)

Gebo [89] explains that the transition from a semiplantigrade foot to a plantigrade foot during evolution had several consequences for the feet of African great apes and humans. These include reduced subtalar mobility and morphological adjustments to the calcaneum and other tarsals. The development of a bony extension along the lateral region of the calcaneum and a prominent peroneal tubercle add stability at heel strike. Also, the calcaneum is permanently fixed in a laterally rotated position in anticipation of being inverted when the heel contacts the substrate. The cuboid is laterally rotated and lifted as is the navicular. It is suggested that the digits of the African great ape common ancestor would resemble those of chimps and gorillas in being relatively short and the big toe, although still abducted, would be less mobile [89]. The modern human and chimp lineages diverged later in the Miocene epoch. Molecular evidence suggests that this took place 5.0 to 8.0 Mya [92–94]. But, foot fossils are poorly represented in the late Miocene higher primate fossil record. Possible candidates for the modern human–chimp common ancestor (hominin–panin common ancestor) include Sahelanthropus tchadensis (6–7 Mya) and Orrorin tugenensis (5.2–5.8 Mya), but there is controversy surrounding the interpretations of the locomotor patterns of these two fossils. Brunet et al. [95] suggested that the forward location of the foramen magnum of S. tchadensis is consistent with it being bipedal and thus it is a hominin rather than the common ancestor of hominins and panins. But others have argued that S. tchadensis cannot be confidently designated a hominin because face and masticatory anatomy and the morphology of the foramen magnum are similar to those of African great apes [96]. Senut et al. [97] cite the external morphology of the neck of the femur as evidence that O. tugenensis was bipedal. The same authors also suggest O. tugenensis was a capable climber as indicated by the curved shaft of a proximal manual phalanx and the pattern of bony crests on the distal humerus. Others claim both the proximal and distal femoral anatomy lack diagnostic bipedal characteristics [98], and Haile-Selassie [99] argues that there is not yet enough evidence to decide whether O. tugenensis is a likely modern human–chimp common ancestor

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or the common ancestor of hominins. A more recent study [100] of the internal structure of an O. tugenensis femoral neck reveals its cortex is distinct from those of the African apes and most like the cortices of the modern human femoral necks, suggesting that it was bipedal. Because we cannot be certain where both S. tchadensis and O. tugenensis are located with respect to the hominin twig of the Tree of Life, we must use other evidence to predict the nature of the hominin–panin common ancestor. Based on parsimony and early hominin foot morphology it would be expected that the hominin–panin ancestor would have feet more similar to chimpanzees than to modern humans. As efficient climbers, brachiators, terrestrial knuckle-walkers, and capable, but infrequent bipeds, chimpanzees possess a mosaic of pedal traits to meet these locomotor demands. Arboreal adaptations in the chimp foot include a talus with a moderately long neck and an asymmetrical trochlear surface, a calcaneocuboid joint designed for pivoting, mobile transverse tarsal joints, an abducted opposable hallux, and long and curved phalanges with prominent flexor sheath ridges [42,89]. These traits are predicted to be among those possessed by the foot of the modern human–chimp common ancestor. Like the African ape common ancestor, the modern human–chimp common ancestor would have had a chimplike plantigrade foot. In chimps, the shape of the trochlear surface promotes medial mobility during dorsiflexion and lateral mobility during plantarflexion, therefore causing the tibia to trace an arcuateshaped pathway as it moves over the stance phase foot. In contrast, the lateral and medial borders of the modern human trochlear surface constrain the tibia to a straighter path as it moves over the foot. The hominin–chimp common ancestor’s leg would have been expected to follow the arcuate pathway, and it is unlikely to have possessed a longitudinal arch. The lack of a longitudinal arch, and the probable presence of a phenomenon called the ‘midtarsal break’ have implications for the gait of the modern human–chimp common ancestor (Figure 1.16. A ‘midtarsal break’ occurs in panins when the lateral side of the foot

Figure 1.16. Bipedal stride in (A) a human and (B) chimpanzee. The ‘midtarsal break’ during the gait of the chimpanzee is noted by the arrow. (Reprinted from Aiello L, Dean C. An Introduction to Human Evolutionary Anatomy, page 508, Copyright 1990, with permission from Elsevier.)

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including the calcaneum and cuboid stays in contact with the ground. As the heel is lifted off the substrate the foot pivots at the calcaneocuboid joint. It is unlikely there would have been any medial weight transfer across the forefoot of the modern human–chimp common ancestor. In the next chapter we focus on the evolution of the foot by reviewing taxa that are almost certainly hominins. We explore the prospect that bipedalism evolved several times, and then trace how the foot of early hominins evolved into a modern humanlike foot.

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2 Recent Evolution of the Human Foot This chapter deals with the recent evolution of the human foot. Its scope is more limited than Chapter 1. Instead of spanning approximately 400 million years it covers only 8 million years, and instead of dealing with the vertebrate branch of the Tree of Life it considers just one of the many small terminal twigs, or clades, that make up the primate part of the Tree of Life. We focus on a series of extinct mammals called hominins that through time, and in fits and starts, accumulate morphology more similar to modern humans and less like that of the other great apes. Researchers have differing views about how many branches the twig has, and they also disagree about how reliably we are able to reconstruct the branching pattern within the twig. We try to reflect these different views in our discussion. This chapter has three sections. The first provides an overview of the hominin fossil record. We use a relatively complex taxonomic hypothesis consistent with human evolution being interpreted as a series of closely related branches called adaptive radiations. A second simpler taxonomic hypothesis that interprets human evolution as a more ‘ladderlike’ progression is presented in summary form. The second section of the chapter reviews hypotheses about the evolution of hominin bipedal locomotion. This focuses on scenarios put forward to explain the origin of bipedalism, and on debates about whether bipedalism evolved once, or several times, within the hominin clade. The third section reviews each hominin taxon, focusing on what is known about the foot. Most of this information is gleaned from fossil foot bones that have, with varying degrees of certainty, been assigned to hominin taxa. However, other information has come from so-called ‘trace fossils,’ which are the impressions made by the feet of early hominins.

Hominin Evolution: An Overview Terminology For reasons given below we treat modern humans as one of the ‘great apes’, the other three being the two African great apes, chimpanzee (Pan) and the gorilla (Gorilla), and the only Asian great ape, the orangutan (Pongo). Palaeoanthropologists have differed, and still do differ in the way they classify the higher primates. We have tried to avoid technical terms in this section, but some are necessary in order to understand the implications of the different classifications. Chapter co-written by Nicole L. Griffin.

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Linnaean taxonomic categories immediately above the level of the genus, that is, the family and the tribe, have vernacular equivalents that end in ‘id’ and ‘in’, respectively. In the past Homo sapiens has been considered to be distinct enough to be placed in its own family, the Hominidae, with the other great apes grouped together in a separate family, the Pongidae. Thus, modern humans and their close fossil relatives were referred to as ‘hominids’, and the other great apes and their close fossil relatives were referred to as ‘pongids’ (Table 2.1A). However, this scheme is inconsistent with morphological and genetic evidence suggesting modern humans are more closely related to chimpanzees than they are to either gorillas or orangutans. In response to the overwhelming evidence that modern humans and chimpanzees are more closely related to each other than either is to the gorilla or the orangutan, some researchers advocate combining modern humans and chimps in the same genus (e.g., [1]), which according to the rules of zoological nomenclature must be Homo. We adopt a less radical solution. We lump all the great apes into a single family, the Hominidae (Table 2.1B). Within the Hominidae we recognise three subfamilies, the Ponginae for the orangutans, the Gorillinae for the gorillas, and the Homininae for modern humans and chimpanzees. The latter subfamily is broken down into two tribes, Panini (or ‘panins’) for chimpanzees, and Hominini (or ‘hominins’) for modern humans. The latter is further broken down into two subtribes: one, Australopithecina, for the extinct primitive hominin genera, and the other, Hominina for the genus Homo, which includes the only living hominin taxon, Homo sapiens. Modern humans and all the fossil taxa judged to be more closely related to modern humans than to chimpanzees are called ‘hominins’; the chimpanzee equivalent is ‘panin’. Thus, modern humans are ‘hominids’ (family), ‘hominines’ (subfamily), and then ‘hominins’ (tribe). We use ‘australopith’ when we refer to taxa belonging to the subtribe Australopithecina.

Defining Hominins Molecular biology has revolutionised our knowledge of the relationships within the great ape clade of the Tree of Life. Relationships between organisms can now be pursued at the level of the genome instead of having to rely on morphology (traditional hard and/or soft-tissue anatomy or the morphology of proteins) for information about relatedness. Comparisons of the DNA of organisms have been made using two methods. In DNA hybridisation all the DNA is compared, but at a relatively crude level. In DNA sequencing the base sequences of comparable sections of DNA are determined and then compared. The results of both hybridisation (e.g., [2]) and sequencing studies of both nuclear and mtDNA (e.g., [1,3,4–6]) are virtually unanimous in suggesting that modern humans and the two African apes are more closely related to each other than any of them is to the orangutan. They also suggest that modern humans and modern chimpanzees (belonging to the genus Pan) are more closely related to each other than either is to the gorilla.

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Table 1 ‘Old’ and ‘New’ Taxonomies (A) A traditional ‘premolecular’ taxonomy of higher primates. Extinct taxa are in bold. Superfamily Hominoidea (hominoids) Family Hylobatidae (hylobatids) Genus Hylobates Family Pongidae (pongids) Genus Pongo Genus Gorilla Genus Pan Family Hominidae (hominids) Subfamily Australopithecinae (australopithecines) Genus Ardipithecus Genus Australopithecus Genus Kenyanthropus Genus Orrorin Genus Paranthropus Genus Sahelanthropus Subfamily Homininae (hominines) Genus Homo (B) A taxonomy of higher primates that recognizes the close genetic links between Pan and Homo. Extinct taxa are in bold type. Superfamily Hominoidea (hominoids) Family Hylobatidae (hylobatids) Genus Hylobates Family Hominidae (hominids) Subfamily Ponginae Genus Pongo (pongines) Subfamily Gorillinae Genus Gorilla (gorillines) Subfamily Homininae (hominines) Tribe Panini Genus Pan (panins) Tribe Hominini (hominins) Subtribe Australopithecina (australopiths) Genus Ardipithecus Genus Australopithecus Genus Kenyanthropus Genus Orrorin Genus Paranthropus Genus Sahelanthropus Subtribe Hominina (hominans) Genus Homo

Most attempts to predict the date of the Pan/Homo dichotomy suggest that the hypothetical ancestor of modern humans and chimpanzees lived between about 5 and 8 Mya (e.g., [7]), but some researchers (e.g., [8]) favour a substantially earlier date (10–14 Mya) for the divergence of the Pan and Homo lineages.

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Thus, if we accept that the hominin twig of the Tree of Life may extend back in time to ca. 8 Mya, and that the earliest unambiguous hominin is probably Australopithecus anamensis (see below), then between 8 and 4 Mya we would expect to find primitive hominin and primitive panin taxa, and close to 8 Mya we should expect to see evidence of the common ancestor of panins and hominins. Not all of these taxa are direct ancestors of modern humans and chimpanzees; many will belong to extinct panin and hominin subclades. Some of these taxa could also belong to extinct clades that have yet to be recognised in the fossil record.

Organising the Hominin Fossil Record The classification of the hominin fossil evidence is controversial. Some researchers favour recognising relatively few taxa, whereas others think that more species and genera are needed to accommodate the observed morphological diversity. We use a relatively speciose (or ‘splitting’) taxonomy, but we also provide an example of a less speciose (or ‘lumping’) taxonomy so that readers can appreciate how the evidence for human evolution would look if it were interpreted in a different way. The taxa included in the two taxonomies are listed in Table 2.2. Some of the taxon names are used in different senses in the speciose and less-speciose taxonomies. When we refer in the text to the hypodigm (the fossil evidence referred to that taxon) of one of these taxa in the speciose taxonomy we use the taxon name followed by sensu stricto (e.g., Au. afarensis sensu stricto or its abbreviation, Au. afarensis s. s.). This signifies that we are using the taxon name in its ‘strict’ sense. When we refer to the same taxon in the more inclusive taxonomy (i.e., the hypodigm is larger) the Linnaean binomial is followed by sensu lato (e.g., Au. afarensis sensu lato or Au. afarensis s. l.). This indicates we are using the taxon name in a ‘looser’ sense. To save repetition, and because in most cases we are referring to the taxa in their strict sense, readers should assume we mean sensu stricto unless we specifically refer to sensu lato, or use s. l. The temporal spans of the taxa in the speciose taxonomy are illustrated in Table 2.2 and Figure 2.1. The ages of the first and last appearances of any taxon in the fossil record (called the ‘first appearance datum’ or FAD, and ‘last appearance datum’ or LAD, respectively) almost certainly underestimate the temporal range of the taxa. Nonetheless, FADs and LADs provide an approximate temporal sequence for the hominin taxa. The heights of the columns of those taxa with good, well-dated, fossil records (e.g., Australopithecus afarensis and Paranthropus boisei) are a reasonable estimate of the temporal range of those taxa, but the heights of the columns for the taxa marked with an asterisk (e.g., Sahelanthropus tchadensis and Australopithecus bahrelghazali) reflect uncertainties about the age

Anatomicallymodern Homo

Archaic Homo

Megadont archaic hominins

Archaic hominins

A. Splitting taxonomy S. tchadensis* O. tugenensis* Ar. ramidus s.s* Ar. kadabba* Au. anamensis* Au. afarensis s. s. K. platyops* Au. bahrelghazali* Au. africanus* Au. garhi* P. aethiopicus* P. boisei s. s. P. robustus* H. habilis s. s.* H. rudolfensis* H. ergaster* H. erectus s. s. H. floresiensis 3 H. antecessor* H. heidelbergensis H. neander thalensis H. sapiens s. s. 7.0 – 6.0 6.0 5.7 – 4.5 5.8 – 5.2 4.5 – 4.0 4.0 – 3.0 3.5 – 3.3 3.5 – 3.0 3.0 – 2.4 2.5 2.5 – 2.3 2.3 – 1.3 2.0 – 1.5 2.4 – 1.6 2.4 – 1.6 1.9 – 1.5 1.8 – 0.2 0.095 – 0.018 0.7 – 0.5 0.6 – 0.1 0.2 – 0.03 0.16 -pres

Age (Ma)

Type specimen TM 266-01-060-1 BAR 1000’00 ARA-VP-6/1 ALA-VP-2/10 KNM-KP 29281 LH 4 KNM-WT 40000 KT 12/H1 Taung 1 BOU-VP-12/130 Omo 18.18 OH 5 TM 1517 OH 7 KNM-ER 1470 KNM-ER 992 Trinil 2 LB1 ATD6-5 Mauer 1 Neanderthal 1 None designated X X X X X X X X X X X X X X

ff X X

X

X

Crania X X X X X X X X X X X X X X X X X X X X X X

Dentition

ff X X

ff X X

(Continued)

X X X

X X ff

X

X

? X X ? X X X

? X X

X ff X X X

X X X X X

X ?

Lower limb

Upper limb

X ?

X

X

Axial

Splitting and lumping hominin taxonomies and skeletal represention1 of splitting hominin taxa

Informal group Primitive hominins

Table 2

Recent Evolution of the Human Foot 31

4.5 – 3.0 3.0 – 2.4 2.5 – 1.3 2.0 – 1.5 2.4 – 1.6 1.9 – 0.018 0.7 -pres

Au. afarensis s. l. Au. africanus* P. boisei s. l. P. robustus*

H. habilis s. l.* H. erectus s. l. H. sapiens s. l.

H. habilis s. s., H. rudolfensis H. erectus s. s., H. ergaster, H. floresiensis H. sapiens s. s., H. antecessor, H. heidelbergensis, H. neanderthalensis

Notes: 1. Skeletal representation key: X – present; ff -fragmentary specimens; ? -taxonomic affiliation of fossil specimen(s) uncertain.

Anatomicallymodern Homo

Ar. ramidus s. s., Ar. kadabba, S. tchadensis, O. tugenensis

7.0 – 4.5 Au. afarensis s. s., Au. anamensis, Au. bahrelghazali, K. platyops Au. africanus P. boisei s. s., P. aethiopicus, Au. garhi P. robustus

Taxa included from splitting taxonomy

Age (Ma)

B. Lumping taxonomy Ar. ramidus s. l.*

(Continued )

Informal group Primitive hominins Archaic hominins Megadont archaic hominins Archaic Homo

Table 2

32 The Human Foot

Recent Evolution of the Human Foot

H. sapiens

H. heidelbergensis H. floresiensis H. erectus H. neanderthalensis H. antecessor 1 H. rudolfensis 0

33

2

P. boisei P. robustus

H. ergaster

Au. garhi H. habilis Au. bahrelghazali

3

P. aethiopicus

Au. africanus Au. afarensis

K. platyops 4 Anatomically modern Homo 5

6

Au. anamensis

Archaic Homo

Ar. ramidus Ar. kadabba

Megadont archaic hominins O. tugenensis

7

Archaic hominins S. tchadensis Possible and probable primitive hominins

8

Millions of Years Ago

Figure 2.1. Speciose hominin taxonomy.

of the taxon because either the sample size is too small, or because the dating methods used are imprecise. For various reasons it is very unlikely that we have a complete record of hominin taxonomic diversity, particularly in the pre-4 Mya phase of hominin evolution. This is because intensive explorations of sediments of this age have only been conducted for less than a decade, and because these investigations have been restricted in their geographical scope. Thus, the fossil evidence we are working with in the early phase of hominin evolution is almost certainly incomplete. More taxa are likely to be identified. We should bear this in mind when formulating and testing hypotheses about any aspect of hominin evolution, especially the evolution of bipedalism. We have not used lines to connect the taxa in the most likely ancestor-descendent sequence, we are reluctant to do this because the constraints of existing knowledge suggest there are only two relatively well-supported subclades within the hominin clade, one for Paranthropus taxa and the other for post-Homo ergaster taxa belonging to the Homo clade. Without more well-supported subclades it is probably unwise to begin to try to identify specific taxa as ancestors or descendants of other taxa. The taxa in Table 2.2 have been assigned to five relatively crude grades [9]; these are based on brain size, postcanine tooth size, and inferred locomotor mode. Several taxon samples are too small to do other than make an informed guess about the grade of the taxon.

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Evolution of Bipedalism within the Hominin Clade One of the most important discoveries relevant to palaeoanthropology in the past century or so has nothing to do with the fossil record. It is the recognition that the chimpanzee genome is more similar to the modern human genome than it is to the genome of the gorilla [10]. This means that the ancestor of all later hominins is itself a descendant of the common ancestor of chimpanzees and hominins. Thus, if we could make reliable inferences about the posture and locomotion of the chimp/human common ancestor this would help narrow down the options for the posture and locomotion of the earliest members of the hominin clade. As we have seen in Chapter 1, comparative evidence suggests that the foot of the chimp/human common ancestor was already adapted for plantigrade locomotion. This is a form of locomotion in which the heel is the first part of the foot to strike the ground at the end of the swing phase [11]. The principle of parsimony suggests that the foot of the chimp/human common ancestor would be more similar to that of living chimpanzees than to the modern human foot. Thus, when seen in the context of the other higher primates it is the posture and locomotion of modern humans that is unusual, not that of chimpanzees. Chimpanzees mostly climb and walk quadrupedally on large branches when they are in the trees. They do brachiate, but it is a minor part of their arboreal behaviour. For most of the time they are on the ground they are quadrupedal knucklewalkers; bipedal terrestrial locomotion is rare (Figure 2.2). The tarsal bones of modern chimpanzees reflect this locomotor mix. The talus has a moderately long neck, an asymmetrical trochlear surface, the calcaneocuboid joint is designed for rotation, and the transverse tarsal joints are designed for mobility. More distally the hallux is abducted and opposable, and the long and curved phalanges have prominent flexor sheath ridges. We predict these traits will be among those possessed by the foot of the chimp/human common ancestor. Rose [12] points out that among primates, and especially higher primates, modern humans are unusual because they are committed to a single locomotor mode. Whereas other primates have a locomotor repertoire made up of several locomotor strategies, modern humans have only one locomotor strategy or mode. This is the form of locomotion we call obligate bipedalism. For example, in modern chimpanzees the two major locomotor strategies employed are climbing and quadrupedal knuckle-walking, with brachiation and bipedalism playing progressively smaller roles in the locomotor repertoire of chimpanzees. Higher primates that have the ability, or facility, to move bipedally are called facultative bipeds, and Rose [12] predicts that in the common ancestor of chimps/humans facultative bipedalism would have established itself as the third major component of its locomotor repertoire, along with climbing and knuckle-walking (Figure 2.3). What we presently do not know is whether this hypothesised relatively modest increase in emphasis on bipedalism was confined to the hominin clade, or whether it also

Recent Evolution of the Human Foot

35

Figure 2.2. Bipedal postures of bonobos (pygmy chimpanzees). These close relatives of humans are only infrequent bipeds; they mainly locomote quadrupedally or by climbing. Note the absence of a longitudinal arch, the divergent or abducted hallux, and the long toes. It is predicted that the chimp/human common ancestor and the earliest hominins would have had similar feet. (From Bernard G. Campbell, James D. Loy. Humankind Emerging, 7/e. Published by Allyn and Bacon, Boston, MA. Copyright © 1996 by Pearson Education. Reprinted by permission of the publisher.)

Chimp/human common ancestor

Archaic hominins Other

Other

Quadrupedalism

Archaic Homo Other Climbing

Climbing Quadrupedalism

Bipedalism

Climbing

Bipedalism Bipedalism

Figure 2.3. Proposed early hominin locomotor repertoires. (Adapted from Rose MD. The process of bipedalization in hominds, in Origine(s) de la Bipédie Chez les Hominidés, Y. Coppens and B. Senut, Editors. 1991, Centre National de la Recherche Scientifique: Paris. p. 40.)

36

The Human Foot

occurred in any nonhominin clade. We also do not know whether this increased emphasis on bipedalism occurred just once within the hominin clade, or whether it occurred independently in an extinct higher primate clade, or in more than one hominin subclade. For various reasons, most researchers consider it is likely that any increase in the importance of facultative bipedalism in early hominins would have been at the cost of quadrupedal locomotion.

Scenarios Favouring Selection for Bipedalism Scenarios explaining the origin of bipedalism can be divided into those that necessitate the adoption of bipedalism as the predominant locomotor mode, and those that favour the adoption of bipedalism. Scenarios that necessitate bipedalism include all the models that involve: (1) the forelimbs being used for prolonged periods for purposes other than locomotion, or (2) the forelimbs being used for any period, short or long, which requires sufficiently radical changes in morphology that they reduce the efficiency of the hands for quadrupedal locomotion or for climbing. The scenarios that necessitate bipedal locomotion include the carriage of infants, food, or tools. The scenarios that favour bipedal locomotion include fast running to evade predators, and more efficient long distance travel between resources that are widely distributed in patches (Figure 2.4). It is also possible that an increased emphasis on an upright, or bipedal, posture may have been an important stimulus for the adoption of a locomotor strategy that included a higher proportion of bipedal locomotion. For example, scenarios that would have favoured an upright bipedal posture include: (1) a shift to a habiSelection for Hominin Bipedalism

Forelimb Pre-emption

Social Behaviour

Feeding

Throwing

Threat/Aggression

Arboreal Gathering

Carrying (e.g., infants, food, tools, etc.)

Evasion of Predators

Terrestrial Gathering

Vigilance

Terrestrial Scavenging

Sexual Display

Aquatic Gathering

Figure 2.4. The three categories, forelimb pre-emption, social behaviour, and feeding may have influenced the adoption of bipedalism by hominins. (Adapted from Rose MD. The process of bipedalization in hominds, in Origine(s) de la Bipédie Chez les Hominidés, Y. Coppens and B. Senut, Editors. 1991, Centre National de la Recherche Scientifique: Paris. p. 41.)

Recent Evolution of the Human Foot

37

tat in which being upright has advantages for thermoregulation [13]; (2) the use of an upright posture for displays, threats, and aggression [14,15]; or (3) the use of an upright posture for arboreal feeding [16]. It is, of course, possible and likely that at any one time more than one of these influences was operating to increase the Darwinian fitness of the members of early hominin taxa that opted to increase the proportion of bipedal travel in their locomotor repertoire. It is also likely that more than one of the influences reviewed above may have been involved at different times during the evolution of early hominin facultative bipedalism. Of all the scenarios that have been put forward for its evolution, the hypothesis that obligate bipedalism is a byproduct of selection for manual dexterity is the most appealing. But as can be seen from the review above, once an upright posture and a bipedal gait had been incorporated into the locomotor repertoire of a hominin taxon, then other factors may well have contributed to it assuming an increasing importance in that locomotor repertoire until eventually facultative bipedalism progressed to obligate bipedalism. Almost all discussions of hominin bipedal locomotion usually make the implicit assumption that early hominins were adapted for bipedal walking. However, two researchers have recently revived an earlier proposal [17] that the selection may have been not for walking, but for bipedal endurance running [18]. The authors suggest that several of the morphological features of the modern human foot are more easily explained as adaptations to running than to walking (ibid., 347). At present the earliest evidence of obligate bipedalism within the hominin clade is seen in Homo ergaster, or East African Homo erectus. Most researchers take the view that hominin obligate bipedalism involves such a complicated set of adjustments to the postcranial skeleton that it is unlikely to have occurred more than once. However, there is good evidence that many hominin adaptations may be homoplasies; that is, they were not inherited from a recent common ancestor. There is also no reason to think that the morphology associated with posture and locomotion is any less prone to homoplasy than, say, the morphology associated with mastication. Thus, it would be unwise to categorically exclude the possibility that facultative bipedalism, or even obligate bipedalism could have evolved more than once in the hominin clade. Therefore we should be cautious before assuming that all later bipedal hominin taxa, be they facultative or obligate bipeds, necessarily inherited their bipedal adaptations from a common ancestor.

Review of Individual Hominin Fossil Taxa Each hominin taxon is placed in one of five informal groupings, ‘primitive hominins’ (P), ‘archaic hominins’ (A), ‘megadont archaic hominins’ (M), ‘archaic Homo’ (AH), and ‘anatomically-modern Homo’ (H) (Table 2.2). Within each grouping the taxa are listed in order of their first appearance in the fossil record.

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The Human Foot

Unless homoplasy (shared morphology not derived from the most recent common ancestor) is even more common than we anticipate, there is little doubt that recent hominin taxa [i.e., post-H. ergaster taxa in group (AH)] are more closely related to modern humans than to chimpanzees. These taxa all have absolutely and relatively large brains; they were obligate bipeds; and they have small canines, slender jaws, and small chewing teeth. However, the closer we get to the split between hominins and panins the more difficult it is to find features we can be sure fossil hominins possessed and fossil panins, or taxa in any other closelyrelated clade, did not. In the early stages of hominin evolution it may be either the lack of panin features or relatively subtle differences in the size and shape of the canines, or in the detailed morphology of the limbs, that mark out hominins. We are conscious that many readers of this book may be unfamiliar with the details of the hominin fossil record, so we provide basic information about the morphology of each taxon. We have deliberately not sorted these features into ‘primitive’ (or plesiomorphic), ‘derived’ (or synapomorphic), and ‘unique’ (or autapomorphic) because this would suggest that hominin cladograms are more reliable than we believe them to be. When a taxon has been moved from its initial genus, the original reference is given in parentheses followed by the revising reference. Further details about most of the taxa and a more extensive bibliography can be found in Wood and Richmond [19]; only selected recent references are cited here. Recent relevant reviews of most of these taxa can be found in Hartwig [20] and Wood and Constantino [21]. Modern and prehistoric carnivores are, and apparently were, partial to hands and feet. Thus there are relatively few hand and foot bones in the hominin fossil record. Clearly this changes when corpses were deliberately buried, but this did not occur until relatively recently, probably within the last 100 Kya. Some hominin fossil foot bones have been found at sites where there is cranial evidence for more than one hominin taxon (e.g., the 2.2 Mya calcaneum, Omo 33.74.896 and the 2.36 Mya talus, Omo 323-76-898, both from the Omo Shungura Formation, two tali, KNM-ER 1464 and 1476, from Koobi Fora and the 1.8 Mya foot fossils from Swartkrans). Because it is impossible to be sure which of the hominin taxa known from these sites these fossil foot bones belong to, we have not included evidence about these remains in any of the taxa listed below. A note about the dates provided for the taxa. Age estimates are given in either millions (Mya) or thousands (Kya) of years.

Primitive Hominins This category includes one taxon, Ardipithecus ramidus, that is almost certainly a member of the hominin clade, one taxon, Sahelanthropus tchadensis, that is a probable hominin, and two taxa, Ardipithecus kadabba and Orrorin tugenensis, that may be hominins.

Recent Evolution of the Human Foot

39

Taxon name: Sahelanthropus tchadensis Brunet et al., 2001. Temporal range: ca. 7–6 Mya. Source(s) of the evidence: Toros-Menalla, Chad, Central Africa. Nature of the evidence: A distorted cranium, several mandibles, and some teeth; no postcranial evidence. Characteristics and inferred behaviour: A chimp-sized animal displaying a novel combination of primitive and derived features. Much about the base and vault of the cranium is chimplike, but the relatively anterior placement of the foramen magnum, the presence of a supraorbital torus, the lack of a muzzle, the small, apically-worn canines, the low, rounded, molar cusps, the relatively thick tooth enamel, and the relatively thick mandibular corpus [22] suggest that S. tchadensis does not belong in the Pan clade. It is either a primitive hominin, or it belongs to a separate, but closely-related, clade of homininlike apes. Pedal fossil evidence: None. Taxon name: Orrorin tugenensis Senut et al., 2001. Temporal range: ca. 6 Mya. Source(s) of the evidence: The relevant remains come from four localities in the Lukeino Formation, Tugen Hills, Kenya. Nature of the evidence: The 13 specimens include three femoral fragments. Characteristics and inferred behaviour: The femoral morphology has been interpreted [24,25] as suggesting that O. tugenensis is an obligate biped, but other researchers interpret the radiographs and CT scans of the femoral neck as indicating a mix of bipedal and nonbipedal locomotion. Otherwise, the discoverers admit that much of the critical dental morphology is ‘ape-like’ [26, p. 6]. O. tugenensis may prove to be a hominin, but it is more likely that it belongs to another part of the adaptive radiation that included the common ancestor of panins and hominins. Pedal fossil evidence: None. Taxon name: Ardipithecus kadabba, Haile-Selassie et al., 2004. Temporal range: 5.8–5.2 Mya. Source(s) of the evidence: Middle Awash, Ethiopia. Nature of the evidence: Mandible, teeth, and postcranial evidence. Characteristics and inferred behaviour: The upper canine and lower first premolar morphology is less apelike than that of O. tugenensis, but more apelike than that of Ar. ramidus. The researchers who found it suggest that it ‘closely approaches the extant and fossil ape condition’ [26, p. 1505], and they also suggest that the morphology of the canine–premolar complex of S. tchadensis, O. tugenensis, and Ar. kadabba is so similar that they may belong to one genus, or even one species (ibid.).

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The Human Foot

Pedal fossil evidence: The only evidence of the foot is a 5.2 Mya fourth proximal phalanx. Haile-Selassie [27] suggests the curvature, length, and dorsal-canting of the proximal articular surface are traits shared with the Pliocene hominin, Au. afarensis. Richmond et al. [28] caution against treating the latter trait as evidence of bipedalism because Duncan et al. [29] did not find a significant difference between the proximal articular surface shape of Au. afarensis and quadrupedal primates such as the African apes. Taxon name: Ardipithecus ramidus sensu stricto (White et al., 1994) White et al. 1995. Temporal range: ca. 4.5–4.4 Mya. Source(s) of the evidence: The initial evidence for this taxon came from a site called Aramis in the Middle Awash region of Ethiopia. A second suite of fossils, including a mandible, teeth, and postcranial bones, recovered in 1997 from five different localities in the Middle Awash that range in age from 5.2–